Practice exercise for working with groundwater contaminant plumes.

Below are two maps of an area with three potential point sources (evaporation ponds). The first map shows the elevation of the groundwater surface in an array of observation wells. The second shows the concentration of a contaminant in samples taken from the groundwater in those wells. What can be deduced from this information?

The first step in the analysis is likely to contour both sets of data. Look at the observation wells as points a continuous surface has been sample. You want to better 'see' the surface. Contouring is a time proven way of 'seeing' the surface better. In this case I would suggest a contour interval of 4 units, and I would suggest starting with the 214 line. A contour separates a portion of the surface (or distribution) that is lower than the given contour value from one that is higher. You simply draw that line separating all the wells with values higher than 214 verus those lower than. Naturally, the 214 contour will pass directly through any wells exactly at 214. Repeat the procedure for 218, 222, 226, and 210 contour lines. Do the same for the contaminant concentration contours. I would suggest contour values of 10, 20, and 30. Note that when you are contouring you have some discretion of exactly where to place the line. This means the contour map is an interpretation. There will be a family of patterns consistent with the data. Remember that shallow aquifer flow will be at right angles to the contours (i.e. directly down slope), and that this should influence the dispersal pattern of any contaminant in the groundwater.

Now you should be able to answer the following questions?

• Which, if any of the point sources may be leaking?
• What is the direction of contaminant transport in the groundwater?
• If the time frame since initial leakage has been 10 years how fast are the contaminants moving?
• Is there a good possibility the contaminants have reached the surface, and what is the evidence for this?